The invention relates generally to methods of recovering and refining dianhydrosugars and, more specifically, to methods for batch, semi-batch and continuous recovery and purification of isosorbide.
The 1,4:3,6-dianhydrosugars, of which isosorbide is an example, are derived from natural products. Therefore, these compounds are classified as “renewable resources.” Furthermore, 1,4:3,6-dianhydrosugars, such as isosorbide, can be used as starting materials and intermediates in chemical reactions. For example, isosorbide is reported to be useful in the production of pharmaceutical compounds, plastic and polymer production, and in other commercial uses such as in the production of polyurethanes, polycarbonates, and polyesters.
For isosorbide to be used as a monomer in high volume polymers and copolymers, for applications such as containers, it needs to be produced in large quantities, preferably in a continuous process and with low operating costs.
A new process for producing isosorbide from sorbitol without the use of a vacuum to remove water formed during the reaction has been developed and is the subject of a co-pending patent application. The temperature and acid strength of the reaction mixture are adjusted to control the amount of water in the reaction mixture to provide conditions for high selectivity and productivity of isosorbide. Crude isosorbide is removed from the reaction mixture via low pressure evaporation and is condensed to produce a concentrated isosorbide solution containing small quantities of high boiling impurities. Typically the quality of crude isosorbide in the evaporator condensate from the enhanced reaction process is in the 97-98 weight percent range (on a water free basis).
U.S. Pat. No. 4,564,692 describes techniques for crystallizing isosorbide and other anhydrosugar alcohols from concentrated aqueous solutions by controlled cooling crystallization from seeded solutions. Unlike the techniques described in this specification, supersaturation was maintained in the range where existing crystals continued to grow without primary or secondary nucleation providing new seed crystals.
U.S. Pat. No. 6,670,033 describes laboratory procedures for purification of anhydrosugar alcohols, including isosorbide. Preferably, the purification is by distillation followed by recrystallization from methanol, ethanol, or ethylene glycol. Employment of water as a solvent was not addressed in this patent.
U.S. patent application Ser. No. 10/414,606 concerns a continuous process for recovering and purifying isosorbide from a water vapor stream. The process is speculative and unproven and no data are presented in the application.
U.S. Pat. No. 7,122,661 describes a sequential treatment technique for trace impurity removal from previously crystallized isosorbide (from organic solvent) involving dissolution in water and treatment with decolorizing carbons and ion exchange resin. The final product from this treatment is a purified isosorbide/water solution. Water is evaporated and the concentrated isosorbide melt is solidified and crushed to produce particulate material.